drm/i915/dpll: drop a useless I915_STATE_WARN_ON()

In general, we don't do assertions that a function gets called on the
right platforms, and if we did, it should not be a state warn.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230512110444.1448231-1-jani.nikula@intel.com